Hydrodechlorination is a process wherein removal of chlorine from chlorine containing compounds such as chloroorganics like chlorofluorocarbons (CFC's) takes place in the presence of hydrogen. Palladium catalysts are preferred for the hydrodechlorination of CFC's in gas phase. Difluoromethne (HFC-32) known as a deep refrigerant is a resultant product of the hydrodechlorination of dichlorodifluoromethane (CFC-12) over palladium based catalysts in vapour phase. Selective formation of HFC-32 by hydrodechlorination of CFC-12 depends on the nature of the catalyst used, the selection of support and the method of preparation which is the black art of the process.
Several methods are known in the art for the preparation of catalysts useful in the hydrodechlorination of CFC-12. Japanese patent application No. 339182JP 0601 731 PCT discloses a process using palladium on activated carbon as a hydrodechlorination catalyst for the production of HFC-32 with a conversion of CFC-12 of 80% and a yield of HFC-32 of 20%. WO 9617683 discloses a process for the production of HFC-32 with a selectivity of ˜80% over palladium—platinum/carbon catalyst. Reference may also be made to another patent application number ENCSM, 34053 Montpellier (CA. No. 119:94820t) which discloses palladium supported on a metal oxide or metal fluoride (Al2O3 or AIF3 respectively) as hydrodechlorination catalyst. The main drawback of oxide supports is the lack of resistance to HF/HCl produced during the reaction. Under this corrosive reaction atmosphere wherein HF/HCl is a by-product, the acidity of the catalyst changes and the oxide support is transformed to hydroxy/oxide fluorides or fluorides resulting in loss of activity. Fluoride supports catalyst side reactions and leads to low yield of the hydrodechlorination product. Also the preparation of fluoride supports involves corrosive reactions due to the use of HF as a reactant. A process for which a patent application has been filed by researchers of the Delft University of Technology, Netherlands reveals high CFC-12 conversion with high selectivity to HFC-32. However, this process is reportedly carried out under a pressure of 0.3–0.5 Mpa. U.S. Pat. No. 5,426,252 discloses the fast deactivation of catalyst comprising of palladium or Group IV B metal carbides. Copending Indian patent application No. 537/Del/99 and 536/Del/99 discloses a process for the use of highly active palladium supported on carbon covered alumina as a catalyst for the hydrodechlorination of CFC-12 with a conversion of 68% and HFC-32 selectivity of 70%. The main drawbacks of the above processes is that of low conversion or low selectivity to HFC-32 or severe operation conditions.
Accordingly, it is important to develop new or modified catalysts, which overcome the disadvantages of the prior art listed above.